In X-ray detectors of the type mentioned initially, such as those which are described in U.S. Pat. No. 2001/0038076 A1 or EP 1 113 290 A2, X-ray converters are used which are composed of a luminescent material, for example of a scintillator or phosphor layer. Such converters first convert an arriving X-ray quantum to a large number of light quanta. A light sensor which is located immediately behind the X-ray converter measures the amount of light arriving, and this is a measure of the X-ray dose arriving at a specific point on the X-ray detector. In this case, semiconductor structures, for example based of silicon, silicon alloys or germanium, with photodiode elements arranged in a matrix are used as light sensors, and can be read via a suitable line structure.
One problem with this type of X-ray detector is the possible creation of “ghost images” owing to what is referred to as the memory effect. In this case, signals from previously recorded images are transferred to later images. This effect is particularly pronounced when X-rays with a high dose have already been produced.
The “ghost images” are in this case created by metastable states, which are referred to as “traps”, being filled within the semiconductor material from which the photodiode elements are formed. These metastable states decay only gradually and produce an additional signal component, the “ghost image”, in subsequent detector read cycles. This occurs in particular with the amorphous silicon which is normally used to form such semiconductor structures. The stronger the input signal, that is to say the stronger the intensity of the light for the photodiodes, the greater the number of electrons that are trapped in the metastable states, and the more significant is the occurrence of ghost images in subsequent X-rays. The image areas of an X-ray which have been overexposed during the relevant X-ray was being taken, that is to say for which the photodiodes in the detector have become saturated and thus do not contain any diagnostic information anyway, thus in particular lead to ghost images.
Software algorithms can be used in order to eliminate these ghost image phenomena, or to reduce them as far as possible. In this case, a reference image or correction image is recorded without any irradiation at specific intervals. The reference image is then subtracted from the subsequent images. However, this method has the disadvantage that the noise component in the image is increased. Furthermore, the normal procedure for an X-ray examination must be interrupted in order to record the reference image.
DE 34 16 716 A1 and DE 31 51 436 A1 describe television cameras in which phototropic filters are associated with the camera objective, in order to extend the exposure freedom.